Research & Related Other Project Information: Section 7 Project Summary/Abstract This pivotal efficacy study is one of two efficacy studies required by the FDA Center for Veterinary Medicine to complete the effectiveness technical section of the new animal drug application. This study is to demonstrate the efficacy of estradiol when administered in feed (5 ppm) to American eels (Anguilla rostrata) to increase the rate of weight. Estradiol will be incorporated into a feed premix (Type A Medicated Article) at the rate of 0.1%. The premix will be top-coated onto fish feed. Eels will be fed to satiation daily with estradiol-medicated feed administered continuously throughout the treatment period starting with wild-caught glass eels that weigh 0.2 to 0.5 g each. The study will consist of 12 experimental units; 6 untreated control units and 6 units that receive estradiol-medicated feed. Each experimental unit will initially consist of 300 glass eels stocked into 1 tank. Eels will be weighed at 35-day intervals and as they grow, eel stocking density will be maintained at optimal levels by a) adding more water into the tank, b) splitting eel numbers into additional tank(s), c) grading so that eels of similar sizes are housed together. A maximum of 4 tanks will be assigned to each experimental unit. All eels will be weighed after 315 days of receiving either estradiol medicated feed or unmedicated feed and the rate of weight gain for each group will be statistically analyzed. Preliminary experiments have shown that continuous administration of estradiol-medicated feed is necessary to produce market weight eels in a commercially feasible period of less than one year.